1. Field of the Invention
This invention relates to a lithium niobate substrate used in surface acoustic wave devices, and more particularly to a lithium niobate substrate that can not easily cause a decrease in yield in device fabrication processes, and a method for producing the same.
2. Description of the Related Art
Lithium niobate (LiNbO3; hereinafter often “LN”) crystals are artificial ferroelectric crystals having a melting point of about 1,250° C. and a Curie temperature of about 1,140° C. Then, LN substrates (hereinafter simply often “substrate(s)”) obtained from LN crystals are chiefly used as a material for surface acoustic wave devices (SAW filters) for removing signal noise of cellular telephones.
The SAW filter (surface acoustic wave device) has a structure wherein thin films of a metal such as AlCu alloy form a pair of comb electrodes on a substrate made up of a piezoelectric material which includes the LN. These comb electrodes play an important role which governs the polarity of the device. Also, to form the comb electrodes, a metal thin film is formed on the piezoelectric material by sputtering and thereafter, leaving a pair of comb patterns, unnecessary portions are removed by etching by a photolithographic technique.
LN single crystals serving as a material for SAW filters are also chiefly obtained by the Czochralski process, in which, usually using a platinum crucible, a crystal is grown in an electric furnace having an atmosphere of a nitrogen-oxygen mixed gas with an oxygen concentration of about 20%, then cooled at a stated cooling rate in the electric furnace, and thereafter taken out of the electric furnace to obtain the crystal.
The LN crystal thus grown is colorless and transparent, or takes on pale yellow color with a high feeling of transparency. After it has been grown, it is, in order to remove any residual strain due to thermal stress of the crystal, subjected to heat treatment under soaking at a temperature close to its melting point, and is further subjected to poling treatment for making it single-polarized, i.e., a series of treatment in which the LN crystal is heated from room temperature to a stated temperature of Curie temperature or more, voltage is applied to the crystal, the temperature is dropped to a stated temperature of Curie temperature or less as the voltage is kept applied, and thereafter the application of voltage is stopped to cool the crystal to room temperature. After the poling treatment, the LN crystal, which has been abraded on its peripheral surface in order to adjust the external shape of the crystal (the one obtained here is hereinafter an “ingot”), is made into a substrate through mechanical working such as slicing, lapping and polishing steps. The substrate obtained finally is substantially colorless and transparent, and has a volume resistivity of about 1015 Ω·cm.
Now, in the substrate obtained by such a conventional method, because of its pyroelectric property which is characteristic of the LN crystal in the fabrication process for the surface acoustic wave device (SAW filter), the substrate surface may come charged up (come full of electric charges) as a result of temperature changes the crystal undergoes in the process. This may produce a spark which makes the cause of destruction of the comb electrodes formed on the substrate surface, and may further cause breakage or the like of the substrate, resulting in a decrease in yield in the device fabrication process.
There is also brought about a problem that, because of a high light transmittance of the substrate, the light transmitted through the interior of the substrate in a photolithographic process which is one of steps in the device fabrication process is reflected on the back of the substrate and returns to the surface to make poor the resolution of a pattern formed.
Accordingly, to solve this problem, as disclosed in Japanese Patent Applications Laid-open No. H11-92147 and No. 11-236298, a method is proposed in which, within the range of from 500 to 1,140° C., the LN crystal is exposed to a chemical reducing atmosphere which is an atmosphere of argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, oxygen, or a gas selected from combination of any of these to effect blackening to thereby restrain the high light transmittance of the substrate and also enhance its electrical conductivity so that the light returning from the back of the substrate can be restrained and at the same time the pyroelectric property can be reduced. Incidentally, as a result of the above heat treatment thus made, the LN crystal, having been colorless and transparent, turns colored and opaque. Then, this phenomenon in which it turns colored and opaque is herein called blackening, because the color tone of the colored and opaque crystal looks brown to black through transmitted light.
Now, in the method disclosed in Japanese Patent Applications Laid-open No. H11-92147 and No. 11-236298, the LN crystal is heated to a high temperature of 500° C. or more. Hence, the treatment time is short on the one hand but on the other hand the blackening tends to come non-uniform between treatment batches, and also there has been a problem that color non-uniformity due to blackening, i.e., in-plane distribution of volume resistivity tends to occur in the substrate having been heat-treated, and the decrease in yield in the device fabrication process can not still be prevented sufficiently.